Security articles have been prepared that include multiple layers of sheeting materials. Sheeting materials having a graphic image or other mark have been widely used, particularly as security labels or laminates for authenticating an article or document. For example, sheetings such as those described in U.S. Pat. Nos. 3,154,872; 3,801,183; 4,082,426; and 4,099,838 have been used as validation stickers for vehicle license plates, and as security films for driver's licenses, government documents, tape cassettes, playing cards, beverage containers, and the like. Other uses include graphics applications for identification purposes such as on police, fire or other emergency vehicles, in advertising and promotional displays and as distinctive labels to provide brand enhancement.
Another form of imaged sheeting is disclosed in U.S. Pat. No. 4,200,875 (Galanos). Galanos discloses the use of a particularly “high-gain retroreflective sheeting of the exposed-lens type,” in which images are formed by laser irradiation of the sheeting through a mask or pattern. That sheeting comprises a plurality of transparent glass microspheres partially embedded in a binder layer and partially exposed above the binder layer, with a metal reflective layer coated on the embedded surface of each of the plurality of microspheres. The binder layer contains carbon black, which is said to minimize any stray light that impinges on the sheeting while it is being imaged. The energy of the laser beam is further concentrated by the focusing effect of the microlenses embedded in the binder layer.
The images formed in the retroreflective sheeting of Galanos can be viewed if, and only if, the sheeting is viewed from the same angle at which the laser irradiation was directed at the sheeting. That means, in different terms, that the image is only viewable over a very limited observation angle. For that and other reasons, there has been a desire to improve certain properties of such a sheeting.
As early as 1908, Gabriel Lippmann invented a method for producing a true three-dimensional image of a scene in lenticular media having one or more photosensitive layers. That process, known as integral photography, is also described in De Montebello, “Processing and Display of Three-Dimensional Data II” in Proceedings of SPIE, San Diego, 1984. In Lippmann's method, a photographic plate is exposed through an array of lenses (or “lenslets”), so that each lenslet of the array transmits a miniature image of the scene being reproduced, as seen from the perspective of the point of the sheet occupied by that lenslet, to the photosensitive layers on a photographic plate. After the photographic plate has been developed, an observer looking at the composite image on the plate through the lenslet array sees a three-dimensional representation of the scene photographed. The image may be in black and white or in color, depending on the photosensitive materials used.
Because the image formed by the lenslets during exposure of the plate has undergone only a single inversion of each miniature image, the three-dimensional representation produced is pseudoscopic. That is, the perceived depth of the image is inverted so that the object appears “inside out.” This is a major disadvantage, because to correct the image it is necessary to achieve a second optical inversion. These methods are complex, involving multiple exposures with a single camera, or multiple cameras, or multi-lens cameras, to record a plurality of views of the same object, and require extremely accurate registration of multiple images to provide a single three-dimensional image. Further, any method that relies on a conventional camera requires the presence of a real object before the camera. This further renders that method ill-adapted for producing three-dimensional images of a virtual object (meaning an object that exists in effect, but not in fact). A further disadvantage of integral photography is that the composite image must be illuminated from the viewing side to form a real image that may be viewed.
Another form of imaged sheeting is disclosed in U.S. Pat. No. 6,288,842 (Florczak et al.). Florczak et al. discloses microlens sheeting with composite images, in which the composite image floats above or below the sheeting, or both. The composite image may be two-dimensional or three-dimensional. Methods for providing such sheeting, including by the application of radiation to a radiation sensitive material layer adjacent the microlenses, are also disclosed. This patent discloses that images are created as a result of a compositional change, a removal or ablation of the material, a phase change, or a polymerization of the coating disposed adjacent to one side of the microlens layer or layers.
US Patent Publication No. 2007/0081254 (Endle et al.) also discloses microlens sheeting with composite images, in which the composite image floats above or below the sheeting, or both. The composite image may be two-dimensional or three-dimensional. Methods for providing such sheeting are also disclosed.
U.S. Pat. No. 5,712,731, “Security Device for Security Documents Such as Bank Notes and Credit Cards,” (Drinkwater et al.) discloses a security device that includes an array of microimages which, when viewed through a corresponding array of substantially spherical microlenses, generates a magnified image. In some cases, the array of microlenses is bonded to the array of microimages.
PCT Patent Application Publication, WO 03/061983 A1, “Micro-Optics For Article Identification” discloses methods and compositions for identification and counterfeit deterrence using non-holographic micro-optics and microstructures having a surface relief greater than a few microns.